The purpose of this research project is to make new high-resolution measurements of basic elastic and contractile properties of cardiac muscle, in order to better understand the process of contraction. Improved apparatus will be used to perform quick-release experiments on right ventricular papillary muscles of the cat and other species. Force and length will be recorded during the course of each release to obtain the force-extension curve of the series elasticity of the muscle. The time course of quick release will be varied, and the effect of release velocity will be systematically studied. Releases will be compared at different times in the twitch in an effort to find elasticity changes reflecting changing myofilament cross-bridge configurations correlated with different stages in the activation sequence. Experimental results will be interpreted in terms of the classical Hill contractionmodel, and more recent cross-bridge interaction models for striated muscle. Differences of cardiac muscle properties from those of skeletal musclewill be noted and interpreted in relation to these models. It is hoped that these studies will provide a better understanding of fundamental processes underlying the normal and abnormal contraction of cardiac muscle, and the distinctive features of these processes in cardiac as compared with other types of muscle.